1. Field of the Invention
The invention relates to an on-load transformer tap changing system used to regulate the output voltage of the transformer secondary by changing the winding ratio. In fact, in numerous applications, the load applied to a transformer may vary and it is nevertheless necessary to maintain a substantially constant output voltage.
2. Discussion of the Background
For this, varying the winding ratio of the transformer is known. These changes are generally made using intermediate taps provided on the secondary or primary of the transformer and using tap changers which are used in this way to modify the winding ratios. These tap changers must function on-load so as not to break the electric current flow. However, the switching of these tap changers induces electrical arcs which are the cause of the degradation of the oil present to provide insulation. Regular maintenance must be carried out to maintain the insulation performances of the fluid.
FIG. 1 shows an example of a transformer tap changing system (OLTC) known in the prior art.
The transformer tap changer comprises an on-load setting switch CX and a selector SE comprising the intermediate taps 1, 2 and 3 of the secondary of the transformer TR.
The taps of the selector set the winding ratios that can be used. The switch CX is designed so as to limit stress during load tap changes.
The setting switch CX comprises a rotary switch CR used to connect an operating output B2 to one of the fixed contacts A to D of the rotary switch. The moving contact of the rotary switch has a sufficient contact surface area to make it possible to connect the output B2 to two fixed contacts next to the rotary switch simultaneously.
In FIG. 1, the rotary switch is in a position connecting the output B2 to the tap 2 of the transformer secondary. To change from the transformer tap 2 to tap 1, it is necessary to turn the rotary switch CR. Said switch first connects the output B2 at the same time to the fixed contacts A and B, and then changes to the fixed contact B thus inserting the impedance ZA into the transformer secondary circuit without breaking the circuit. Then, the moving contact connects the output B2 to the fixed contacts B and C. The load taps 1 and 2 are both connected to the output B2 via the impedances ZA and ZB respectively. Then, the moving contact connects the output B2 to the fixed contact C, i.e. to the transformer tap 1 via the impedance ZB, and then to the two fixed contacts C and D. Finally, it connects the output B2 to the fixed contact D thus only connecting the output B2 to the tap 1.
Therefore, the change of transformer load taps (from tap 1 to tap 2) is made without breaking the transformer secondary circuit. Any other tap change would result in similar sequences.
Therefore, the electrical circuit is never open during a tap change by providing a transient state where a portion of the transformer winding is short-circuited.
In addition, to prevent a prohibitive current, impedances ZA and ZB are placed in series in the circuit.
However, when the moving contact switches to the fixed contacts A to C, electrical arcs may appear on the contacts, which represent a drawback as mentioned above.
FIGS. 2a and 2b represent a type of on-load transformer tap changer known in the prior art and used to prevent the formation of electrical arcs during tap switching. This changer uses semiconductor switching circuits using gate turn-off (GTO) thyristors and mechanical switches used to reduce the tap changing time in the absence of an electrical arc.
The principle of this selector is similar to that described above but the switch is modified: the resistors and the rotary switch are replaced by semiconductor switching circuits IN1, IN2, IN3, an auxiliary transformer tra and mechanical switches S1 to S5.
The circuit comprising the auxiliary transformer tra and the switching circuit IN2 provide, as described, for example, in the document EP0644562, the permanent connection of the output terminal B2 to a tap of the secondary of the transformer TR.
The switching circuits IN1 to IN3 are produced as represented in FIG. 2b. Each switching circuit comprises four diodes and a gate turn-off thyristor.
In FIG. 2a, if it is assumed that the system is such that the contacts S2 and S4 are closed and the switching circuit IN2 is conductive, the power supply from the transformer TR is supplied via the tap 2. If the winding ratio is to be modified and the system switched so that the power supply is provided via the tap 1, the system in FIG. 2a will complete the following process:                closure of the switch S1,        detection of the zero transition of the load current and once said current passes via zero, opening of the switching circuit IN2 and closure of the switching circuit IN1. A few moments later, the switch S4 is opened when the magnetic current of the auxiliary transformer passes through it,        detection again of the zero transition of the load current, closure of the switching circuit IN3 and opening of the switching current IN1,        closure of the switch S5 while the current is not zero,        detection again of the zero transition of the load current, closure of the switching circuit IN2 and opening of the switching current IN3. The circuit is now connected to the tap 1 of the transformer.        
This operation is illustrated by the timing diagrams in FIG. 2c. In these diagrams, the operation of each contact and each switching circuit of the system in FIG. 2a is individualised by a specific diagram. For the contacts S1 to S5, the top sections of the diagrams represent the closed positions of the contacts, the bottom sections represent the open positions of the contacts, and for the switching circuits IN1 to IN3, the top sections represent the conductive states of said circuits and the bottom sections, the non-conductive states.
In the bottom section of FIG. 2c, the current flowing in the secondary winding of the transformer TR is represented. This is necessary because the switching of the switching circuits IN1 to IN3 must be carried out in the absence of current flow or possibly at a very low or negligible current.
Therefore, it can be seen that this system has the drawback of requiring the detection of the zero transition of the load current whenever the state of the switching circuits IN1 to IN3 is to be changed so that the switching of these circuits is carried out at the lowest current possible.
It should be noted that the switching time of the switches S1 to S5 is markedly greater than the switching time of the switching circuits IN1 to IN3.
In addition, the gate turn-off thyristors provided in the switching circuits IN1 to IN3 require limitation of the voltage variations on the terminals of said thyristors during the switching thereof. As represented in FIG. 2b, a resistor-capacitor type circuit CN is then provided to control the voltage variations at the thyristor terminals and an inductive resistor in series with the resistor reduces the current variation rate. The size of these RC circuits and of the inductive resistors is linked with the amplitude of the switched current.
In addition, the trigger current applied to the gate G and necessary to control the thyristor turn-off is proportional to the switched current.
Therefore, the system in FIGS. 2a and 2b involves the drawback of requiring circuits associated with the thyristors to limit the voltage and current of these components.
In addition, as described above, a load current zero transition detection circuit must be provided. The drawback of this solution also lies in the reliability of the equipment associated with the need for a load current zero transition detection circuit.
In addition, the use of such a control principle for a three-phase application induces a transitory imbalance during the changes. In fact, the current is not zero in the three phases simultaneously. Therefore, the switching of the current of each of the phases is not simultaneous and one detection circuit per phase must be used.